Heya!
So, in the wild it's thought they can live well over 50 years.
In captivity, the oldest known captive crab is Jonathan Livingston Crab (you can google him) who has been in captivity for 40 years.
This is pretty darn uncommon!! They don't do super well in captivity - but if you end up with a relatively healthy crab & keep it in good conditions, it is possible to get 10 years, and sometimes more from them... there are several members on here who've had crabs a decade or so - I hope they'll chime in.
Colour is influenced by many factors, including diet & horomones, and who-knows what else.
It is true that crabs seem to darken as they age (I've seen older PPs that are almost black) but this can happen in smaller crabs too. You get the occasional bigger crab who is super pale, too.
Captive crabs tend not to grow as large as their wild counterparts. Size can give us a vague idea of age, but isn't reliable... nutrition, genetics, dominance & resource access may all play a part in size.
It's likely that most of the crabs we get in stores are
at least 5-10 years old.
Hermits start off reeeeal small. Kind of mid-size spider-sized, really. People are gradually having more success breeding them, and in a few years we'll have a better idea of how long it takes them to reach the 'small', 'medium', and 'large' sizes. Look up the breeding threads on here to see how adorably teensy they are when they first crawl up on land - and how little their shells are... [explanation devolves into high pitched cooing and baby talk...]
Anyway, for now, the answer to your question is... guess.
Luckily, crabs won't notice if you put the wrong number of candles on their birthday cake.... I don't think they're very good at counting!
Edit - being slow may be a sign of getting g ready to moult. It can also result from lower temperatures. Or just being a lazy-bones.